1. Field of the Invention
The present invention relates to a collision detection system used for an airbag system or the like in a vehicle. The present invention also relates to an acceleration sensor for use with a moving object such as a vehicle for detecting the acceleration or deceleration of the moving object and more particularly, to a piezoelectric acceleration sensor comprising a pendulum supported for movement relative to a housing which is fixed to the moving object and a piezoelectric element which detects the displacement of the pendulum relative to the moving object to generate a voltage in proportion to the relative displacement. The term "acceleration sensor" as used herein refers to a sensor which senses both acceleration and deceleration.
2. Description of the Prior Art
Automobiles are required to be as simple as possible in maneuravability and yet provide pleasurable driving and these demands necessitate ingenious controls for various parts of the automobile. Recent advanced electronic control techniques have been introduced to the automobile industry. It is necessary for the control of the automobile to have operational data such as the vehicle running state and operational states of various parts of engine, and thus, various sensors are provided on a vehicle for this purpose.
A prior art collision detection system, as disclosed in U.S. Pat. No. 4,164,263, comprises an acceleration sensor which uses a strain gauge pickup and a signal processing circuit connection to the output of the acceleration sensor, wherein in operation, if the acceleration sensor detects an acceleration greater than a preset value due to the collision of the vehicle, the signal processing circuit produces an output to activate a security device such as an airbag.
The conventional collision detection system, including the acceleration sensor itself, must undergo an inspection for abnormalities such as the failure of the acceleration sensor and/or wire breakage in the signal processing circuit. This inspection is performed by applying a mechanical vibration to the acceleration sensor and measuring the output of the signal processing circuit with an instrumentation apparatus. However, this diagnostic procedure to the collision detection system is too intricate for the vehicle user to carry out.
Acceleration sensors for detecting the acceleration of moving objects are broadly classified into the following types: the piezoelectric type, dynamic electric type, servo type, and strain gauge type. Among these types of sensors, the piezoelectric sensor is most commonly used in the field of general acceleration measurement, and the other types of sensors are used mainly in special instrumentation fields.
A typical piezoelectric acceleration sensor consists of a pendulum movably supported relative to a housing which is fixed to a moving object under measurement and a piezoelectric element having a face joined to the pendulum. The sensor operates such that the piezoelectric element is distorted in response to the displacement of the pendulum, resulting in the generation of a voltage in proportion to the acceleration of the moving object.
The use of such a piezoelectric acceleration sensor is exemplified by the operation circuit of an airbag system, as disclosed in Japanese Patent Publication No. 58-22377. This piezoelectric acceleration sensor is designed to detect the deceleration of a vehicle and produce a voltage signal in response to the deceleration in excess of a preset value. The signal ignites a detonator in the airbag system so that the airbag is inflated to hold a passenger in the seat.
The foregoing conventional piezoelectric acceleration sensor functions solely to detect the acceleration and thus, for inspection of the sensor a mechanical vibration or impact must be applied to the sensor by means of a special test device such as a vibrator. Generally, this kind of test device is too bulky to be provided in average automobiles such as passenger cars. Therefore, the car user is obliged to detach the sensor from the vehicle and take it for inspection to a specific facility equipped with a diagnostic vibrator.